The present invention relates to video systems. More specifically, the present invention relates to low vision video magnifiers that accommodate people with vision impairments.
Video magnifiers are often used to assist persons with xe2x80x9clow visionxe2x80x9d [2] to read printed materials, [2] to view written information while writing, and [3] to look at photographs. Physicians and other health care providers usually use the term xe2x80x9clow visionxe2x80x9d to describe chronic vision problems that generally cannot be corrected through the use of glasses or other lens devices, medication or surgery. Moreover, symptoms associated with low vision frequently result from a degeneration or deterioration of the retina of a patient""s eye, a condition commonly referred to as macular degeneration and associated with older individuals.
FIG. 1 illustrates a video magnifier 100. Such video magnifier 100 is commercially available from Telesensory Corporation located in Sunnyvale, Calif.; Telesensory Corporation is also the assignee of this present application. The video magnifier 100 has a reading platform 10 on top of a base 11 to accommodate a magazine 12, printed materials or photographs. The reading platform 10 may be stationary or slidable along x and y axes. Once a user places the magazine 12 upon the reading platform 10, an image capturing device (not shown) within the video magnifier 100 captures images on a page of the opened magazine. Thereby, the video magnifier 100 displays the captured images to the user at an enlarged view. Additionally, the user may also place a check onto the reading platform 10 and thereby view the enlarged written information while writing.
To further accommodate people with vision impairments or low vision, the video magnifier 100 includes a video polarity control lever 14, which is partially shown and is similar to a lever 15, for the user to adjust and thereby see positive and negative video images of the captured images. FIG. 2A illustrates a viewing area of the image capturing device represented by a dotted box 20 within which printed information of, for example, the magazine 12 of FIG. 1 locates. The image capturing device captures such printed information within the dotted box 20. Thereby, depending on the position of the video control lever 14, the video magnifier 100 displays the captured, printed information at an enlarged view having [1] positive video images as shown in FIG. 2B when the user has chosen the positive video polarity or [2] negative video images as shown in FIG. 2C when the user has selected the negative or reverse video polarity. Regarding the selection of the positive and negative video polarities, the positive video polarity is typically selected for viewing photographs, coins, stamps, labels of medicine bottles, and so on. The reason is that there is minimal or no movement of the viewed matters because such movement may yield blurred images especially when the total area of white is more than the total area of black. However, a user with low vision must rely substantially on his or her peripheral vision to see and thus tends to get eye fatigue from viewing the displayed positive video images for a long period of time because the glare produced by the large white area eventually affects the peripheral vision. With respect to the selection of the negative or reverse video polarity, it should be selected when the user wants to read, e.g., a book because the displayed negative video images effectively shows more black than white so as to reduce eye fatigue. However, reading text requires the user to move the book back and forth and, as a result, this movement causes motion blur especially when levels of brightness and contrast of the displayed images are typically set very high to accommodate people with vision impairments.
To capture the printed information, the video magnifier 100 includes a lens and camera assembly (not shown). The video camera of the lens and camera assembly typically contain a charge-coupled device (CCD) and possibly contain a CMOS (complementary metal-oxide semiconductor) image sensor. Although CCD cameras can operate in both fixed and variable shutter modes, the CCD camera of and within the conventional video magnifier 100 captures images in either [1] the fixed shutter mode or [2] the variable or automatic shutter mode, but does not capture images in both modes. In other words, the CCD camera of the conventional video magnifier 100 utilizes only the fixed or variable shutter mode to capture images for both the positive and negative video polarities selections.
The CCD has an exposure time or shutter opening time for each picture frame. This exposure time may be fixed or varied automatically by a circuitry of the camera as a function of the average value of light intensity or brightness falling on the pixel cells of the CCD sensor. During the exposure time, the pixel cells are allowed to accumulate an electronic charge representing a particular picture frame. The value of the electronic charge depends on the light intensity or brightness of the image due to the local illumination and, to some extent, the ambient illumination falling on the reading material.
To ensure that there is sufficient brightness provided upon the magazine 12 for the camera to capture the images, the video magnifier 100 relies primarily on its local illumination and may also rely on the ambient illumination. With respect to the local illumination, the video magnifier 100 typically includes and uses at least one fluorescent lamp or incandescent lamp. Generally, the fluorescent lamps are chosen because they have relatively uniform spectrum and cast a fairly even distribution of light over the reading platform 10 upon which the magazine 12 is placed. With respect to the ambient illumination, fluorescent lamps at the ceiling of a room in which the video magnifier 100 is being operated typically provide such ambient illumination. For both local and ambient illuminations, the fluorescent lamps used may be driven by the ac power line via [1] a magnetic ballast circuit or [2] a high frequency power oscillator (electronic ballast).
When the fluorescent lamps are driven by the a-c power line via the magnetic ballast circuit, their output illumination has a steady state component 28 and a pulsating component 30 as illustrated by FIG. 3B. The pulsating component 30 has a ripple frequency equivalent to twice the ac power line frequency, which may be 50 hertz or 60 hertz, as illustrated by FIG. 3A. This is true for the local fluorescent lamps utilized by the video magnifier 100 as well as for the ambient fluorescent lamps at the ceiling of the room. When the ac power line frequency is different from that of a frequency of 59.94 hertz of a vertical field rate of the camera, an interaction or interference occurs between the frequencies of the output illumination and the vertical field rate and, as a result, a beat frequency is formed. As the difference between the ac power line frequency and the vertical field rate frequency increases, the beat frequency also increases and is more noticeable. With respect to the video magnifier 100, this beat frequency is naturally more noticeable in the displayed video images because the levels of brightness and contrast of the display device of the video magnifier 100 are typically set very high to accommodate users with vision impairments. In particularly, when the positive video polarity is selected and thereby positive video images are displayed, the beat frequency is especially noticeable because the large area of white representing the paper of the magazine 12 is flickering or winking its brightness at the user while the user is viewing such displayed images. In contrast, when the negative video polarity is selected, the beat frequency is less apparent because there is more black area than white area and thus the flickering or winking is less evident.
When the fluorescent lamps are driven by the ac power line via the high frequency power oscillator, their output illumination does not flicker or wink since the drive frequency chosen will be high enough to allow the persistency of their lamp phosphor to filter out the illumination ripple. Although this type of arrangement is most desirable because its steady illumination produces no interaction with the vertical field rate, it is not always utilized because high frequency power oscillators are expensive.
It is an object of the present invention to provide a video system that captures images and thereby displays the captured images with negligible or without any flickering/winking effect and image motion blur from the perspective of a user who views the captured images.
It is another object of the present invention to provide a video system that allows a user to manually select the operation of an image capturing device of the video magnifier in either a fixed or variable shutter mode.
According to one embodiment of the present invention, a video system for capturing images via its image capturing device is provided. The video system has a selection device allowing a user to manually select the operation of the image capturing device in either a fixed shutter mode or a variable shutter mode. During the fixed shutter mode operation, the image capturing device of the video system has an exposure time that is one half of a period of a cycle of the ac power line. The video system may also have a second selection device for allowing the user to manually select between positive and negative video polarities so as to view positive and negative video images, respectively.
According to another embodiment of the present invention, a video magnifier is provided for capturing images [1] in the fixed shutter mode when the user selects the positive video polarity and [2] in the variable shutter mode when the user selects the negative video polarity. During the fixed shutter mode operation, an image capturing device of the video magnifier has an exposure time that is one half of a period of a cycle of the ac power line.
According to another embodiment of the present invention, a method of capturing images for display in positive and negative video polarities is provided. First, an image capturing device operable in fixed and variable shutter modes is provided. Second, either the positive or negative video polarity is selected. And finally, the images are captured in the fixed shutter mode when the positive video polarity is selected and in variable shutter mode when the negative video polarity is selected. During its fixed shutter mode operation, the image capturing device has an exposure time that is one half of a period of a cycle of the ac power line.